1. Field of the Invention
This invention relates, in general, to computer program security and, more particularly, to cryptographic microprocessors.
2. Description of the Background
Methods and apparatus for preventing firmware piracy are many and varied. One method of preventing firmware piracy is found in the Best patent (U.S. Pat. No. 4,278,837) issued on July 14, 1981. The Best patent discloses a method of preventing program piracy in which programs are stored in a ciphered form. A cryptomicroprocessor then deciphers the programs step-by-step as the program is executed. However, in the Best patent, each program is encrypted using a different variable and, as a result, does not facilitate use of low cost, mass production devices, such as masked ROMs. An alternative, to allow mass production of the Best approach, is to encrypt each program using a single common variable. This method prevents duplication of the plain text version of the program but does not prevent simple duplication by a pirate of the ciphered text. Since all of the programs have a common variable the copied ciphered text may be used on any microprocessor that is encrypted in the same manner.